When doing a Graceful Failover is there a way of finding out what Unique Keys have been used before? This will, hopefully, not happen too often and the chances are we will have forgotten what keys we have used in the past so it would be useful if we could find out somehow what has been used.

Dbvisit Standby will complain if you use a key that has been used before. So you do not have to remember which keys you have used before.

To give you some background info:
Graceful Switchover stores the used keys in the temp location as X.dbvisit.KEY.*.lck Where KEY is the number that you entered. Before Graceful Failover, you can remove all these files (on both primary and standby) and all the history of the previous keys used will be gone.

BTW, any key sequence is fine, so 8883455, or 78 or 837 or 7475abc is all fine.